This invention relates to an improved eddy current rotary probe type testing device adapted for rapidly scanning the interior walls of tubes or holes in metal parts in order to detect faults such as fissures, cracks or other defects. A highly useful application is to inspect fastener holes in metal aircraft parts wherein strength or fatigue of structures are highly critical and the number of holes to be tested and their varying accessibility to apply the test, pose worker fatigue and performance problems. A broad object of this invention is to provide a compact, readily portable and consistently reliable test device of this nature suitable for such applications. More specifically it is an object for such purposes to provide an easily manipulated hand-held device that provides precise indications of fault location and magnitude even under varying operating conditions and when used by operators of limited skills and experience. The invention is herein illustratively described by reference to the presently preferred embodiment thereof; however, it will be recognized that certain modifications and changes with respect to details may be made without departing from the essential features involved.
While the principles of the invention may be applied to the detection of faults in surfaces other than the walls of fastener holes, this latter application is presently considered to be the most important and is therefore chosen as the basis for describing the invention.
A further object hereof is to provide such a test device that may be utilized rapidly and efficiently in testing a large number of holes over a relatively short time period and of doing so with a high degree of sensitivity and reliability of the indicated results.
A further advantage of this invention, which it shares with certain prior art rotary probe eddy current type testers, is that it incorporates sensing transformer elements in the probe differentially connected so as to cancel out and reject from the sensing circuit the effects of false responses due to transitional positioning of the probe, such as those that can occur when the probe is being inserted in and removed from a test hole. These transitional effects are sometimes designated by the characterizing terms "lift-off," "balance," or "directional edge" effects.
The present invention also utilizes differentially connected probe-mounted sensing transformer secondaries, but in this case they are deliberately slightly mismatched so as to provide a uniquely definitive output response. This invention also contemplates indication sweep circuit improvements of unique value to the bulk, weight and versatility requirements of the major objectives.
With battery power used to operate the circuits and also to drive the probe motor of the improved device, the unavoidable contacts of the rotating probe with the hole wall produce drag that can reduce the speed by a substantial percentage. Furthermore, battery voltage also drops with aging and prolonged usage. With these variable conditions in view, a further object hereof is to provide a fault detection indicator that depends in part for its consistency of results on an improved indicator sweep circuit operable independently of probe speed or, for that matter, battery voltage also. A number of prior art synchronized sweep circuits may be used in principle to provide sweep voltages of equal magnitude despite variable sweep recurrence frequency, examples include those discussed in the following designated U.S. Pat. Nos. 2,532,576; 3,002,153; 2,604,515; 3,292,092; 2,924,707; 3,323,353; 2,948,857; 3,577,007.
However, the present sweep system has specialized advantages, primarily in its low cost, compactness and reliable simplicity for broad purposes of the invention.
Still another object of this invention is to provide an improved fault detection system with complemental indicators, one a graphic display that delineates the specific character and location of the fault, and the other an alert display that prominently signals the presence of a fault so as to direct the operator's attention to the graphic display. As a result the operator need not strain in needless continuous concentration on the graphic (i.e., cathode ray tube) indicator over prolonged time periods while many holes are being successively tested. Furthermore, since the operator need not divert his attention from carefully positioning the test probe in successive test holes in order to study the relatively complex displays of the CRT indicator at every stage, the work becomes much less tiring and the results better on the average over the time of a work shift.